Manufacture of electrical circuits

ABSTRACT

A method of providing an electrical circuit wherein a carrier, which is a film of insulating plastic material with a circuit pattern thereon is supported in a mould and a moulding material is applied by the application of heat and pressure to provide a substrate so that the circuit is embedded in or within a three-dimensional surface of the moulded substrate.

This is a division of application Ser. No. 07/071,267, filed July 1,1987now U.S. Pat. No. 4912288.

BACKGROUND OF THE INVENTION

This invention relates to the manufacture of electrical circuits formedon a surface of a substance of insulating plastics material

Modern, so-called "engineering plastics", are increasingly being used asparts, sub-assemblies or structural members in all kinds of manufacturedgoods; such as motor cars, aircraft, "white" goods, telecommunicationsequipment There is often a requirement to incorporate an electricalcircuit on such parts and, hitherto, this has been achieved by printing,bonding or otherwise applying the circuit onto the surface of such partsto effectively produce a "printed" circuit on an external surface of athree-dimensional object The printed circuit may include integrallyformed resistors, capacitors and inductors and may also have otherdevices added to the circuit.

Electrical circuits of this kind can only be applied to an externalsurface of an object, are prone to physical damage and can exhibit theundesirable characteristics of inter-conductor contamination byhandling, solder flux residues and accumulated debris, such as dust, inuse, which will all reduce the insulation resistance and dielectricproperties or give rise to variation in these properties.

U.S. Pat. No. 4,415,607 (Denes et al); assigned to the Assignee of thisapplication, discloses a method of making a printed circuit networkdevice including the steps of depositing an insulating primary substratelayer (a screen printed layer of titanium dioxide loaded epoxy resindissolved in a combined butyl Cellosolve acetate and n-butanol solvent)on a temporary support structure having a release surface (asuper-calendered, clay-coated one side natural kraft paper, coated onthe clay side with a silicone release coating), applying to the exposedsurface of the primary substrate layer certain defined conductor areas,(screen printed conductive ink using the same epoxy resin based vehicleloaded with a conductive material), mounting the temporary supportmember with its several layers (together forming a decalcomania) infacing relationship with a permanent support member, and thereafterreleasing and removing the temporary support member from the mountedprimary support layer.

The sole given application is for the production of printed circuitnetwork devices incorporating resistive paths and, for this, the methodadditionally comprises the steps of applying defined resistor areas onthe exposed surface of the primary substrate layer and in electricalconnection with the conductor areas and trimming the respective resistorareas to a predetermined resistive value. An insulating permanent baseor support being injection moulded directly to the decalcomania.

SUMMARY OF THE INVENTION

The present invention is an extension and adaptation of the method ofthe above-identified application with the object of embedding a circuitpattern of electro-conductive or electro-resistive material in or withinany surface of a three-dimensionally shaped substrate of insulatingplastics material According to the present invention, a method ofproviding an electrical circuit on a surface of a three-dimensionallyshaped substrate of insulating plastics material, comprises thesequential or non-sequential steps of:

(i) providing a carrier, which is a film of insulating plasticsmaterial, with a circuit pattern of electro-conductive orelectro-resistive, heat-resistant synthetic resin applied to at leastone face thereof;

(ii) supporting a face of the carrier;

(iii) forming the patterned carrier into a given three-dimensionalshape; and

(iv) moulding, by the application of heat and pressure, a substrate ofinsulating plastics material against the unsupported face of thecarrier;

so that the circuit is embedded in within a three-dimensional surface ofthe moulded substrate.

Surfaces with embedded circuits produced by this method have theadvantages of the circuit being protected against physical damage,improved or constant electrical characteristics (because there is novolume between proud circuit elements to accumulate debris of one formor another).

According to one embodiment of the present invention, athree-dimensionally formed, patterned carrier is inserted in a mouldcavity with a face of the carrier supported on a surface of the mouldcavity having a shape complementary to that of the formed carrier andthe mould cavity is then charged with an insulating thermo-formableplastics material which forms the substrate.

According to another embodiment of the present invention, a mouldedsubstrate, having the circuit embedded in or within surface thereof, ispost-formed to a given three-dimensional shape; the circuit material andthe substrate material being thermo-plastic.

According to a further embodiment of the present invention, the circuitpattern is applied to one face of the carrier, which face has a releasesurface for the circuit pattern material, and the carrier is removedfrom the moulded substrate to leave the circuit embedded in thesubstrate, flush with a surface thereof With this embodiment there is ahigh metal density at the exposed surface of electro-conductive circuitparts, thus improving lead connection thereto.

According to yet a further embodiment of the present invention thecircuit pattern material, the carrier material and the substratematerial are all compatible so that the carrier is bonded with themoulded substrate.

Thus, the circuit may be embedded flush with a surface of the substratewith the carrier as an outer, insulating layer bonded to the carriersurface; or, the carrier may be embedded in the substrate with thecircuit embedded in the carrier, the carrier having been deformed duringthe substrate moulding process to conform to the three-dimensional shapeof the circuit and form an effectively flush outer surface with thecircuit; or, the carrier may be provided with a circuit pattern on bothfaces whereby one circuit is embedded in the carrier, substantiallyflush with the outer surface thereof, and the other circuit is embeddedin the substrate beneath the carrier, by choice of dielectriccharacteristics for the carrier material, the two circuits can combineto form capacitors with through vias provided to interconnect the twocircuits, multi-layer assemblies can be built up using this process, aninsulating layer having to be provided between each "pair" of circuits;

or, with the moulded substrate and embedded circuit or circuitspost-formed to any desired shape, the circuits can be embedded in theinner surfaces of complexly shaped mouldings.

The substrate, with embedded circuit or circuits, can be of any shapecapable of being produced by conventional injection moulding processes.Features such as holes or protuberances of any form could beincorporated during the moulding process for fitment of additionalpieces to the moulding to form an assembly or a sub-assembly foraddition to a higher level assembly. Using either thick and/or thin filmtechnology it is also possible to incorporate conductors, resistors,capacitors, filter networks and inductors to be inlaid in the substratesurface by the method of the present invention.

It is also possible, using the method of the present invention, toproduce multi-layer devices by forming alternate, further layers ofsubstrate and circuits; with interconnections between the layers asnecessary.

Some advantages given by the present invention are:

single component construction;

the method is solely additive;

robust construction;

and,

heat sinks can readily be incorporated

The above and other features of the present invention are illustrated,by way of nonlimiting example, in the Drawings, wherein:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a decal;

FIG. 2 is a perspective view of the decal of FIG. 1, after moulding;

FIG. 3 is a perspective view of a moulded substrate;

FIG. 4 is a section on the line A-A of FIG. 3, prior to removal of thetemporary carrier;

FIG. 5 is a perspective view of a decal prior to insertion in a mould;

FIG. 6 is a perspective view of the decal of FIG. 5, with a moulded-onsubstrate;

FIG. 7 is a perspective view of the substrate of FIG. 6, with anembedded circuit;

FIG. 8 is a perspective view of the substrate of FIG. 5, afterpost-forming;

FIG. 9 is a perspective view of a patterned carrier

FIG. 10 is a section, on the line B--B of FIG. 9, of a substrate with acircuit embedded "face-down";

FIG. 11 is a section, on the line B--B of FIG. 9, of a substrate with acircuit embedded "face-up";

FIG. 12 is a section, on the line B--B of FIG. 9, of a substrate withcircuits embedded both "face-down" and "face-up"; and

FIG. 13 is a sectional sketch of a patterned carrier inserted in a mouldcavity, prior to moulding the substrate of FIG. 12.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown by FIGS. 1 to 4, a decal 1 consists of a circuit pattern 2 ofelectro-conductive or electro-resistive ink applied to a face 3 of athin, flexible film 4 of insulating, thermo-formable material Thesurface of face 3 is treated or otherwise formed so as to be able torelease the circuit pattern 2 and any moulding formed thereon, asdescribed below.

Suitable inks are generally an epoxy resin base vehicle loaded withconductive or resistive material, such as silver powder or ferric oxide;e.g. Heraeus CERMALLOY (Trade Mark) CL80-5231. A suitable carriermaterial is General Electric ULTEM (Trade Mark) film, thermoplasticpolyetherimide. The CERMALLOY ink is screen printed on the ULTEM filmand is left uncured; curing being effected by further processingAlternatively, a thermoplastic ink may be employed.

The decal is then formed, by any suitable thermo-forming technique, intothe shape shown in FIG. 2 The formed decal is mounted on a mould core ofcomplementary shape (not shown), the decal and core are then insertedinto a mould cavity and a component having the shape shown in FIG. 3 isthen injection moulded onto the formed decal, the core supporting thenon-patterned face of the decal during the moulding process; producingthe component 5 shown in FIG. 4. Finally, the temporary carrier 3 isstripped from the component moulding 5 to leave the circuit 2 embeddedin the inner surface 5 of the component A suitable material for thecomponent is Du Pont RYANITE (Trade Mark) FR 530 NC-10, a 30%glass-reinforced modified PETP, thermo-plastic polyester resin.Alternatively, and as described in more detail below, a non-releasesurface carrier may be provided and left bonded to the substrate.

FIGS. 5 to 8 illustrate an alternative, continuous manufacturing processwherein a decal 10, generally similar to decal 1 of the above-describedembodiment, is employed; this decal 10 being a continuous film 11 withmarginal, indexing sprocket holes 12 and a series of circuit patterns 13printed on the under, release face 14 of the film. For simplicity ofillustration, the circuit pattern 13 is shown to be an X, although morecomplex patterns would, in reality, be employed In this embodiment, asuitable carrier film is Du Pont KAPTON (Trade Mark) film, a thermosetpolyimide.

The decal film is shown in FIG. 1 to be indexed between the top die 15and the bottom die 16 of an injection mould, wherein a simplerectilinear moulding 17 (FIG. 6) is formed against the underface 14 ofthe decal by the cavity 18 in the bottom die 16; the flat face (notshown) of the top die 15 supporting the non-patterned face of the decalduring the injection moulding process The mould runner 19, formed by theinlet 20 to the mould cavity 18 (FIG. 1) at this stage remainingattached to the moulding 17. The temporary carrier film 11 is thenremoved to leave the circuit 13 embedded in the moulding, flush with thesurface 21 thereof

Thereafter the moulding 17 is post-formed to any desired shape byconventional thermo-forming techniques; e.g. hot forming or vacuummoulding (both the circuit material and the moulding material arethermo-plastic). In the example illustrated by FIG. 8, the moulding 17is inserted between a hot internal punch 22, having a rectilinear shape,and a hot die 23, having a cavity 24 of rectilinear shape complementaryto that of punch 22 The moulding 17 is formed into an open topped box 25having the circuit 13 embedded in the internal box surface 26. Asuitable material for the moulding 17 again being RYANITE 530-NC-10.

As shown by FIG. 9, a circuit pattern 31 of electro-conductive and/orelectro-resistive material is applied to one face 32 of a carrier 33 inthe form of a thin, flexible film of insulating material Again,CERMALLOY ink is screen printed on to ULTEM film and left uncuredAlternatively, a thermo-plastic ink may be employed

FIG. 10 shows one embodiment of substrate, with a circuit embedded"face-down", to consist of a carrier 33 with applied circuit pattern 31and a substrate 34 injection moulded onto the patterned face 32 of thecarrier; to leave the circuit 31 embedded flush with the surface 35 ofthe substrate and the carrier 33 bonded to the substrate surface 35, toform an insulating cover for the circuit and substrate surface Thesubstrate is formed by inserting the patterned carrier in a mould cavity(not shown) with the non-patterned face 36 supported by a surface of themould cavity, so as to present the patterned face to the interior of thecavity Insulating, thermo-plastic material is then injected into themould cavity, the heat and pressure of the injection moulding processbonding the injected material to the carrier surface 32 and around thecircuit 31 The circuit material, the carrier material and the substratematerial all have to be compatible A suitable substrate material,compatible with CERMALLOY ink and ULTEM film, is ULTEM resin, anamorphous thermo-plastic polyetherimide

FIG. 11 shows another embodiment of substrate, but with the circuitembedded "face-up", this embodiment is generally similar to the FIG. 10embodiment but it is produced by inserting the carrier 33 into the mouldcavity with the patterned face 32 supported by a surface of the mouldcavity, so as to present the non-patterned face 36 to the interior ofthe cavity During injection moulding the carrier 33 is shaped to conformto the three-dimensional shape of the circuit 31 and the cavity surfaceso that the circuit becomes embedded in the carrier substantially flushwith the outer face thereof; the substrate 34 bonding to the inner face36 of the carrier.

FIGS. 12 and 13 show a further embodiment of the substrate, this timehaving circuits "face-down" and "face-up", wherein the carrier 33 isprovided with a circuit pattern 37 on face 36 in addition to the pattern31 on face 32. The carrier is inserted in the mould cavity 38 with itspatterned face 32 supported by cavity surface 39. Resin material isinjected into the cavity, as with the FIG. 10 and 11 embodiments, theheat and pressure created during the moulding processthree-dimensionally shaping the carrier 33 about circuit 31, to embedcircuit 31 substantially flush with the outer face 32 of the carrier andembed circuit 37 in the carrier surface 35, beneath the shaped carrier33.

Clearly, by providing a compatible insulating interlayer, multi-layered"pairs" of circuits can be built up using this process.

In an alternative, unillustrated moulding process, the embodiments ofFIGS. 10, 11 and 12 are produced by providing pre-moulded substrates ofthermo-plastic material, laying the patterned carrier onto the substratesurface and applying heat and pressure (such as by a heated roller) tothe carrier. Orientation of the patterned face of the carrier relativeto the substrate determines whether circuits would be embedded"face-down" or "face-up". This method of moulding lends itself to acontinuous process.

Generally, the carrier material has a higher melt temperature than thesubstrate material when the carrier is to be removed from the mouldedsubstrate and of the same or similar melt temperature when the carrieris to be bonded to the substrate. The inks may be uncured thermo-setresins or thermo-plastic resins.

We claim:
 1. A method of providing an electrical circuit in athree-dimensionally shaped surface of insulating plastics material,comprising the sequential or non-sequential steps of:(i) providing acarrier, which is a film of insulating plastics material, with a circuitpattern of electro-conductive or electro-resistive, heat-resistantsynthetic resin applied to at least one face thereof; (ii) supporting aface of the carrier; (ii) forming the patterned carrier into a giventhree-dimensional shape; and (iv) moulding, by the application of heatand pressure, a substrate of insulating plastics material against theunsupported face of the carrier;so that the circuit is embedded in asurface of the moulded substrate forming the three-dimensional surface.2. A method as claimed in claim 1, wherein a three-dimensionally formed,patterned carrier is inserted in a mould cavity with a face of thecarrier supported on a surface of the mould cavity having a shapecomplementary to that of the formed carrier and the mould cavity is thencharged with an insulating thermo-formable plastics material which formsthe substrate.
 3. A method as claimed in claim 2, wherein the circuitpattern material, the carrier material and the substrate material areall compatible so that the carrier is bonded with the moulded substrate.4. A method as claimed in claim 3, wherein the carrier is inserted inthe mould with a patterned face of the carrier constituting at leastpart of the surface of the mould cavity; to produce a moulded substratewith the circuit embedded flush with a surface thereof.
 5. A method asclaimed in claim 4, wherein the carrier is bonded to and covers saidsubstrate surface.
 6. A method as claimed in claim 1, wherein a mouldedsubstrate, having the circuit embedded in a surface thereof, ispost-formed to a given three-dimensional shape; the circuit material andthe substrate material being thermo-plastic.
 7. A method as claimed inclaim 1 wherein the circuit pattern is applied to one face of thecarrier, which face has a release surface for the circuit patternmaterial, and the carrier is removed from the moulded substrate to leavethe circuit embedded in the substrate, flush with the surface thereof.8. A method as claimed in claim 7, wherein, after the carrier has beenremoved, the moulded substrate is post-formed to a giventhree-dimensional shape.
 9. A method as claimed in claim 1, wherein amoulded substrate of thermo-plastic material is provided, a face of thecarrier is applied to a surface of the substrate and heat and pressureare applied between the carrier and the substrate; to produce a mouldedsubstrate with the circuit embedded in the substrate surface.
 10. Amethod as claimed in claim 9, wherein the patterned face of the carrieris applied to the substrate with the circuit embedded in the substrateflush with the surface thereof.
 11. A method as claimed in claim 10,wherein the circuit pattern is applied to one face of the carrier, whichface has a release surface for the circuit pattern material, and thecarrier is removed from the moulded substrate to leave the circuitembedded in the substrate, flush with the surface thereof.
 12. A methodas claimed in claim 11, wherein, after the carrier has been removed, themoulded substrate is post-formed to a given three-dimensional shape. 13.A method as claimed in claim 10 wherein the carrier is also ofthermo-plastic material and the moulded substrate is post-formed to agiven three-dimensional shape.
 14. A method as claimed in claim 9,wherein the carrier is of a thermo-plastic material and thenon-patterned face of the carrier is applied to the substrate surface;to produce a moulded substrate with the circuit embedded in andsubstantially flush with the carrier face and the carrier shaped aboutthe circuit and embedded in the substrate surface.
 15. A method asclaimed in claim 14 wherein the moulded substrate is post-formed to agiven three-dimensional shape.
 16. A method as claimed in claim 9,wherein a circuit pattern is applied to each face of the carrier andeither patterned face is applied to the substrate surface; to produce amoulded substrate with one circuit embedded substantially flush with aface of the carrier, the carrier shaped about the circuit and embeddedin the substrate surface and the other circuit embedded within thesubstrate surface beneath the carrier.
 17. A method as claimed in claim16 wherein the carrier is also of thermo-plastic material and themoulded substrate is post-formed to a given three-dimensional shape. 18.A method as claimed in claim 9 wherein the carrier is also ofthermo-plastic material and the moulded substrate is post-formed to agiven three-dimensional shape.
 19. A method for moulding an electricalcircuit package wherein a flexible support member is secured to asubstrate comprising:placing an electrically conductive circuit tracematerial on one side of a thin flexible film material; positioning saidflexible film material in a mould; and introducing a resinous plasticmaterial into said mould in a manner to fuse with said flexible filmmaterial.
 20. The method as claimed in claim 19 wherein said mould is ofthe injection mould type and said resinous plastic is injection mouldedagainst said flexible film.